The need for extrusion billets with high quality surfaces to minimize waste in the extrusion process has been recognized for many years. A useful technique for achieving such quality has been to use an open-ended casting mould having a refractory inlet section, i.e. a "hot top", forming an overhang below which the molten metal spreads out to form a meniscus in the resulting corner of the overhang, and to solidify while in contact with the chilled mould surface below the overhang. It has been mould that the injection of gas and liquid lubricant into the area below the overhang in such mould gives beneficial results to the surface of the ingot.
Mitamura et al, U.S. Pat. No. 4,157,728 discloses the injection of gas into the upper corner of the mould, immediately below the overhang, with lubricant generally injected at a point slightly below the gas. The gas and oil were both delivered to the mould face by means of small holes or channels drilled in the mould from a delivery channel or ring to the mould face. The gas flow was adjusted to cause a slight pressure increase and thereby force the meniscus slightly away from the corner. The lubricant flowed down the mould face in the usual manner. This method of delivery of gas kept the gas and lubricant generally isolated from each other and further, a very stable and finely adjustable gas flow control was needed to control the gas pressure.
In Wagstaff et al, U.S. Pat. No. 4,598,763 there is disclosed the use of a porous graphite ring which forms at least part of the mould wall below the overhang. Gas and liquid lubricant are delivered to grooves behind the ring or in the surface of the ring opposite the mould surface, and mixed within the ring to form a gas-lubricant mixture which spreads out over the surface of the ring to provide such a mixture in contact with the solidifying ingot. This approach permits higher gas delivery pressures behind the ring and simplifies the control of gas flow. However, the oil gradually fills the porosity in the graphite rings particularly if it reacts because of the heat from the solidifying casting, and its flow control characteristics therefore change in time. The graphite must be carefully selected to ensure uniform delivery of the gas-lubricant mixture.
Steen et al, Australian Application AU-50653/96 discloses the use of separate porous rings or wall members forming part of the mould wall in the above mould type. The two porous wall members are separated by an impervious seal. Gas is introduced through the lower porous member and liquid lubricant through the upper. The lower member therefore remains uncontacted by oil except on the casting face and retains its control properties for longer periods of time. The construction of such mould with two porous wall members and an intermediate seal is more complex, and the use of a porous wall member for lubricant delivery can still result in slow changes to its characteristics. For example, it may be necessary to increase the lubricant oil delivery pressure. Lubricant filled wall members will also require periodic replacement.
There is, therefore, a need for a simple and reliable hot top mould design permitting gas and lubricant delivery below the overhang, which is easy to control and can operate for long periods between maintenance.
It is an object of the invention to provide a direct chill casting mould permitting gas and lubricant delivery to the casting surface which has stable gas and lubricant supply over several casts.
It is a further object of the invention to provide a direct chill casting mould permitting gas and lubricant delivery to the casting surface which is easily maintained.